The present invention pertains to rotary drill bits, and particularly to fixed-cutter bits generally termed "drag bits" in the industry.
There are basically two types of cutting actions achievable with a fixed cutter on a rotating drag bit, the first being a shearing or scraping action, commonly generated by the use of a planar cutter having a circular or other arcuate profile on the part of the cutter contacting the formation, and a plowing or kerfing type action, commonly generated via the use of a polyhedron-shaped cutter oriented with a point or edge projecting above the face of the bit.
The planar cutters currently in use are generally formed of a planar layer polycrystalline diamond on a supporting substrate and are commonly called "PDC's", while the kerfing type of cutters are self supporting thermally stable polycrystalline diamond structures ("TSP's") in the shape of a disc or polyhedron. The former type of cutter must be affixed, as by brazing, to a tungsten carbide matrix type of drill bit after the bit is furnaced, since the PDC's are extremely degraded if not totally destroyed by the bit furnacing temperature employed. The latter type of cutter, TSP's, are so-called because they can survive the bit furnacing operation without degradation.
It has been proposed to simulate a large PDC type planar cutter utilizing a planar mosaic like array of TSP's, thereby permitting planar cutters to be furnaced into the bit in a single operation. Such cutters are disclosed in U.S. Pat. No. 4,726,718, assigned to the assignee of the present invention, the disclosure which is incorporated herein by this reference.
Large, planar TSP cutters similar to PDC's have recently become available on the market. While such cutters can be furnaced into a matrix-type bit, their cost is extremely high, and economics dictate sparing use thereof.
One problem confronting PDC cutters, individual TSP cutters, mosaic-type TSP cutters and the newly-introduced large planar TSP cutters, is the dulling of the cutters as the drill bit wears during drilling, causing the bit weight to be applied to an ever-increasing cutter area as the PDC or large TSP cutters flatten and the pointed TSP cutter points wear. The TSP "mosaic" planar array cutters suffer the same dulling problems as the PDC's.
There has been an appreciation in the industry that a cutter which is self-renewing would be desirable, but there has been no success in achieving such an end result.